Oxide ion conductivity, phase transitions, and phase separation in fluorite-based Bi 38-x Mo 7+xO78+1.5x

Abstract

We present, for the first time, the ionic conductivity properties of two different, but closely related, bismuth molybdates: Bi38Mo 7O78 and Bi37.5Mo7.5O 78.75. Both are good oxide ion conductors, with the latter being comparable to yttria-stabilized zirconia. We show that the structure of Bi 38Mo7O78 is more complex than previously reported, and that this compound is a 5×3×6 fluorite superstructure with slight monoclinic distortion. In addition to being a good oxide ion conductor, the material is noncentrosymmetric-polar and second harmonic generation (SHG) active. The second phase, orthorhombic Bi37.5Mo 7.5O78.75, reported for the first time, is an excellent oxide ion conductor. The materials have been characterized by impedance spectroscopy, variable-temperature synchrotron, neutron and laboratory powder X-ray diffraction, electron diffraction, and SHG measurements.